If you seem to be missing some songs, its possible that they didnt download properly when you purchased them. In this case, try selecting Check for Purchases, from the Advanced menu. This will allow you to download any songs that didnt completely download when you bought them.
If you see an exclamation point icon appear next to your songs, this means your songs were accidentally moved or deleted. Finding lost songs can be a little tricky, so if youre new using computers, you may want to get some help. If you want to try finding songs that were just moved, you can try to re-link them up. If your iTunes Library file was deleted or corrupted, try dragging your files back into iTunes to rebuild your library.
The best way to ensure you dont lose your music and videos is to perform regular backups. To recover songs and videos from a data CD or DVD, just insert the CD or DVD, and drag the songs and videos back to your iTunes Library.
When your iTunes Library file has been deleted or corrupted, rebuild your library by just dragging your song files into iTunes.
Find Music on the iTunes Music Store
Your iTunes Music Store Account
Find Videos on the iTunes Music Store
Find Audiobooks on the iTunes Music Store
Authorizing and Deauthorizing Your Computers
Protecting Your Media and Troubleshooting